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Structural heterogeneities and mechanical properties of polymeric systems

Physical modellization of mechanical behaviour of amorphous polymers

François Lequeux, Hélène Montes, Peiluo Shi

The behaviour of rubber which does not crystallize when subjected to a given elongation depends on the application time of the solicitation, and the strain. The question of balance between the stress softening and stress hardening is of fundamental importance for the optimization of properties of polymeric materials such as toughness. The objective of this study is to define the micro-structural parameters that optimize the energy dissipation of the material at high speeds and large deformations. This will test to what extent it is possible to verify the principle of time-temperature equivalence associated with the existence of the rupture envelope. The proposed approach is to expand the width of the spectrum of relaxation time in order to understand the effect of the coexistence of plastic zones and elastic zones in one sample. This is done by blending miscible polymers with large gap of Tg. The relaxation time spectrum of polymer blends is obtained by rheological and DSC measurements. We use an infrared camera to measure the self heating of samples at high deformation. The high strain rate is performed by the Split-Hopkinson bar.

SBR : time-temperature equivalence and self heating.

Towards the preparation of model filled rubbers with polystyrene-butadiene and polyisoprene

François Lequeux, Hélène Montes, Dongmei Yang

In tire industry, silica filled rubbers has permitted the obtaining products with tailored physical and mechanical properties, such as improving the rolling resistance and wet traction without reducing wear resistance. The objective of this project is to better understand, from the fundamental point of view, the interface between silica and elastomer (polystyrene-butadiene or polyisoprene) in order to finely tune the rubber properties, like viscoelastic and mechanical properties, and to develop a better modeling, from the molecular scale up to the macroscopic scale, in order to help devising the best filler dispersions and surface treatments. The crucial parameter that determines the mechanical properties of the filled rubber is the choice of the silica particles shapes and their dispersions. If the distance between particles surface is too small, the polymer confined in between these two surfaces becomes glassy and stain-soften under mechanical solicitations, which leads to Payne effect. In this project, colloidal spherical silica nanoparticles prepared by Stöber method are used as filler. Neutron scattering characterization is applied to investigate the quality of the dispersion of silica particles in the rubber matrix, and solid nuclear magnetic resonance (NMR) allows us to quantitative measurements on the polymer dynamics at the filler/matrix interface.